reference : https://mailchimp.com/help/set-default-merge-values/#:~:text=Click%20the%20Manage%20Audience%20drop,re%20done%2C%20click%20Save%20Changes.

1. Click the Audience icon.
2. Cursor Clicks - Audience icon
3. Click Audience dashboard.
4. If you have more than one audience, click the Current audience drop-down 
   and choose the one you want to work with.
5. Click the Manage Audience drop-down and choose Settings.
6. On the Settings page, click Audience fields and *|MERGE|* tags.
7. In the fields in the Default value column, type the default values for your 
    merge tags.
8. When you're done, click Save Changes.